Generally, this kind of derailleur includes a derailleur body mainly comprising a linkage mechanism of a support member, a pair of linkage members, and a movable member, and a chain guide pivoted to the movable member. The derailleur is disposed axially outwardly of the minimum tooth number high speed sprocket at a multistage sprocket assembly mounted through the support member to the bicycle frame supporting a rear wheel. The chain is stretched across a front gear at a crank means and one of the multistage sprockets engages with the tension pulley and guide pulley. The movable member is shifted by a control wire toward the maximum tooth number low speed sprocket against a return spring at the derailleur body and by a restoring force of the return spring toward the minimum tooth number high speed sprocket, so that the chain is adapted to be switched to any desired particular sprocket.
This kind of derailleur is provided with an energy conservation mechanism to enable a preparatory speed-change operation by an operating lever even when the chain is not driven by pedalling, and then the chain, when driven by the pedalling, can be switched to the preset speed-change stage by the lever.
The normal pedalling for forward movement of the bicycle enables smooth switching of the chain, but in a case where the control wire is operated to preshift the chain guide from the low speed sprocket to the high speed sprocket, especially when the chain engages with the maximum tooth number low speed sprocket, the chain guide, which usually is biased by the return spring toward the minumum tooth number high speed sprocket, becomes shifted thereto to a maximum and the chain is inclined between the maximum tooth number low speed sprocket and the guide pulley.
Accordingly, under this condition, when the bicycle wheel is rotated in reverse and the multistage sprocket assembly rotates in reverse, the chain, while engaging with the maximum tooth number low speed sprocket, is driven therefrom toward the guide pulley. As a result, the chain is intended to be slack such that it curves outwardly from the guide pulley rather than engaging therewith. Then, the chain is largely slack and leaves the chain guide. The chain also loses it holding force for holding the chain guide at the position corresponding approximately to the maximum tooth number low speed sprocket, whereby the chain guide is shifted by the return spring to the position corresponding to the minimum tooth number high speed sprocket.
Thus, the chain curves outwardly from the guide pulley and this leads to shifting of the chain from the path of proper movement of the chain.
Hence, in this condition, when the multistage sprocket assembly normally rotates by normal rotation of the wheel or by normal pedalling, the chain, which curves outwardly and comes out from the proper chain line between both the pulleys, is pulled by the maximum tooth number low speed sprocket, at which time the chain happens to fall between the maximum tooth number low speed sprocket and the pulley plate at the multistage sprocket assembly side, thereby creating a problem in that the rear derailleur and other parts there around may be broken.
On the contrary, in a case where the control wire is pulled when the chain engages with the high speed sprocket and the chain guide is preshifted from the high speed sprocket side to the low speed sprocket side, when the bicycle is moved rearwardly to rotate the sprocket assembly in reverse, the chain is raised in the direction of disengaging from the guide pulley, thereby creating a problem in that the chain may fall down similarly to the above.
On the other hand, in a case where the multistage sprocket assembly is rotated in a reverse direction by reverse pedalling other than the backward movement of the bicycle, the chain carried between the multistage sprocket assembly and a front chain gear travels at the upper side or slack side toward the rear sprocket and is driven at the lower side or tight side from the tension pulley toward the front chain gear. As a result, the chain guide is put in condition of turning forwardly upwardly at the tension pulley side.
For example, in the state where the chain engages with the low speed sprocket, the chain guide, when preshifted toward the high speed sprocket side, is positioned slantwise between the low speed sprocket and the guide pulley. Under such condition, the reverse pedalling causes the chain to leave the guide pulley and be locked between the low speed sprocket and the pulley plate so that if the reverse pedalling continues in this condition, the derailleur leads to a breakdown.